@mabaroshi
When you say your right hand gets, do you mean your strumming hand? I ask because you could be a left handed player and your right hand is your fretting hand.

If it is your strumming hand, it could be that you have to build up stamina. You might also want to take a close look at how you have the guitar positioned. If the position causes you to put your strumming hand in an awkard or uncomfortable position, try moving your guitar up or down by adjusting your guitar strap. You could also try anchoring your hand with your pinky on pickguard. If you are anchoring, try not anchoring.

Perhaps it's because of the style of music I play (blues and rock), but I've never had an issue with my strumming hand getting tired. My fretting hand...well that's another story!

With my fretting hand, I have found that the guitar itself plays a large part in it. With my strat, I find my hand gets tired rather quickly when doing my scale exercises; nowhere near as much with my les paul. The neck on my strat is longer and wider than my lp and my hands are on the smaller side, so this contributes. I still work through it on my strat because I figure it essentially acts as a stretching and stamina exercise for me. Ultimately, the lp is best suited to my physical makeup.